A typical wireless communication system includes a number of base stations each radiating to provide coverage in which to serve wireless communication devices (WCDs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices, whether or not user operated. In turn, each base station may sit as a node on a core access network that includes entities such as a network controller, switches and/or gateways, and the core network may provide connectivity with one or more external transport networks such as the public switched telephone network (PSTN) and the Internet. With this arrangement, a WCD within coverage of the system may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other WCDs served by the base station.
Such a system may operate in accordance with a particular air interface protocol, examples of which include, without limitation, Long Term Evolution (LTE) (using orthogonal frequency division multiple access (OFDMA) and single-carrier frequency division multiple access (SC-FDMA)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), IEEE 802.11 (WIFI), and BLUETOOTH. Each protocol may define its own procedures for registration of WCDs, initiation of communications, handover between base station coverage areas, and other functions.
Further, each base station may provide wireless service to WCDs on one or more carrier frequencies, with the air interface defining various downlink and uplink channels. For instance, on the downlink, the air interface may define a reference channel for carrying a reference signal (also referred to as a pilot signal, or by other names) that WCDs can measure to evaluate base station coverage quality, the air interface may define various other downlink control channels for carrying control signaling to WCDs, and the air interface may define one or more downlink traffic channels for carrying bearer data and other information to WCDs. And on the uplink, the air interface may define an access channel for carrying WCD access requests to the base station, the air interface may define one or more other uplink control channels for carrying control signaling to the base station, and the air interface may define one or more uplink traffic channels for carrying bearer data and other information to the base station.
When a WCD initially enters into coverage of such a system, the WCD may scan for a best base station coverage area in which to operate, and the WCD may then engage in signaling with and via the base station, to register for service. In particular, the WCD may evaluate a reference signal receive quality (RSRQ) of each base station reference signal that the WCD detects, where the RSRQ defines a signal-to-noise ratio of the reference signal (e.g., a ratio of reference signal receive power to noise power, also referred to as SINR, SNR, C/I, Ec/Io, or by other names), and the WCD may select the coverage area having the highest RSRQ. Alternatively or additionally, the WCD may evaluate another coverage quality metric, such as reference signal receive power (RSRP), of each base station reference signal, and the WCD may select the coverage area having the best (e.g., highest) such metric, perhaps in combination with the highest RSRQ. The WCD may then engage in signaling with the associated base station, to register for service.
The WCD may then be served by the base station in a connected state or an idle state. In the connected state, the WCD would have an established radio-link-layer connection with the base station through which the WCD and base station could exchange bearer data (e.g., application-layer communications), facilitating communication by the WCD on an external transport network for instance. And in the idle state, the WCD would not have an established radio-link-layer connection with the base station but may periodically check for paging from the base station and may transition to the connected mode when necessary to engage in bearer communication.
When so served, the WCD may also regularly monitor the quality of coverage from its serving base station and from any adjacent base stations (e.g., base stations providing coverage overlapping with that of the serving base station), to help ensure that the WCD is served with suitable coverage and perhaps with the best available coverage. In particular, the WCD may regularly evaluate coverage quality (e.g., RSRQ and/or one or more other coverage quality metrics) from its serving base station and coverage quality from each adjacent base station. And if the coverage quality from the WCD's serving base station becomes threshold low and the coverage quality from an adjacent base station becomes threshold high (e.g., threshold higher than from the serving base station), then the WCD may hand over to be served by the adjacent base station.